In many cases it is difficult to control the friendly fire hazard when the 90 degree FOV is combined with operating in a close formation. This is especially important in an urban setting where the limited situational awareness increases the risk of such accidents.
1. The subject is not about IFF. It is about friendlies running into your line of fire or not knowing that you have just thrown a grenade into a room.
2. If somebody breaks from your formation and runs 5 metres in front of you without giving any warning, the weapon lock detects when he is dangerously close to the firing line and stops the shooter. A certain range threshold needs to be in place and 15-30 metres would be sufficient as IFF is not an issue there.
3. The normal rules for IFF will apply if an unidentified person is in your sights at say 100 metres and if you make a mistake you can potentially kill a friendly (weapon lock will not interfere).
4. Finally a 3-dimensional circle grenade indicator can highlight where a friendly grenade is thrown to units in sufficient proximity. This greatly helps when voice chat is unavailable for one reason or another. The indicator is mostly beneficial in clearing buildings as it will ensure that each player has the warning not to enter a particular room.